Cubic boron nitride (CBN) is a crystalline material prepared by subjecting pyrolytic or hexagonal boron nitride to high temperatures and pressures to transform the crystalline structure to a more highly ordered structure having improved hardness and thermal properties among other differing properties. CBN crystal particles and other materials, such as diamond and silicon carbide particles, are known in the art to be useful as abrasive materials because of their hardness and thermal properties. These particles are particularly useful when they are distributed in a bonding matrix and the system containing the matrix with the abrasive particles is used as an abrasive component in a tool. The abrasive particles are dispersed in the bonding matrix which is then fired or hardened by known methods to securely retain the particles throughout the matrix. One of the desired characteristics of the abrasive particle-containing matrix systems is good retention of the particles in the matrix in order that the abrasive particles are held in a working contact position during use. Such matrices containing abrasive particles are particularly useful as the abrasive component in tools used, for example, for grinding, polishing or machining hard materials, such as metals or ceramics. For instance, the matrix-abrasive particles systems can be used to make grinding wheels.
One type of bonding matrix which has found great utility is a vitreous, i.e. a glass based, bonding matrix. These matrices exhibit the good wear resistance and thermal stability necessary for the matrix systems. Unfortunately, some problems have been encountered in the art when trying to use CBN particles in vitreous bonding matrices. The vitreous bonding matrices which are useful in these systems are reactive with CBN and generally contain alkali metal oxides which are also reactive with CBN. The reactiveness of the matrix with CBN is detrimental to the system in that an excessive amount of nitrogenous gas by-product, or ammonia if water is present, is given off under sintering or use conditions which causes bloating at the interface of the matrix and the CBN particle. These phenomena lead to bloating and slumping, i.e. loss of shape, of the desired matrix form. Such form inconsistencies caused by bloating are detrimental to the use of the matrix in abrasive tools, for example, grinding wheels may lose their shape due to bloating.
Consequently, the art has turned to the practice of coating CBN particles with metals or metal oxides or organic materials when using them in a vitreous bonding matrix, as in U.S. Pat. No. 4,951,427 and the patents discussed therein. Such "barrier" coatings are employed in order to prevent the reaction of CBN with the matrix. However, CBN particles coated with a metal, metal oxide or organic compound provide only physical bonding between the CBN particle and the coating and between the coating and the matrix.
Better retention of the CBN in the matrix would be very desirable and would provide better abrasive components. Particularly, it would be desirable to have an abrasive system wherein the CBN particles were chemically bonded to the matrix. It is well known in the art that chemical bonds provide a much stronger bonding force than mere physical bonding. However, the problem of bloating when using chemically reactive coatings has turned the art away from the possibility of chemically bonded CBN.